Amplifier circuits



Nov. 24, 1942. s. HUNT 2,302,866

AMPLIFIER CIRCUITS Filed Dec. 1, 1939 I I0 701. a) T E L 210 sou/20s 20577' p r0 PRIOR SIGNAL GRIDS) $7,552

E LIIII A VC RECTIFIER :Z 60 T0 6/2/05 OF PRIOR TUBES INVENTOR.

SEYMOUR HUNT Patented Nov. 24, 1942 2,302,866 AMPLIFIER CIRCUITS SeymourHunt, Jackson Heights, N. Y., assignor to Radio Corporation of America,a corporation of Delaware Application December 1, 1939, Serial No.307,001

'7 Claims.

My present invention relates to signal amplifier circuits, and moreparticularly to amplifier tube circuits adapted to have signal voltageinduced in electrode circuits by the action of the electrons within atube.

In the past it has been disclosed that a socalled virtual cathode eifectcan be produced within a tube provided with at least a cathode, negativeinput grid, a positive screen grid, a negative suppressor grid and apositive output electrode. The virtual cathode efi'ect appears to existbetween the screen grid and the suppressor rid. The efiect may beattributed to the slowing up of the electrons, emitted by the cathodeand attracted by the screen grid, by the suppressor grid. The density ofthe virtual cathode varies with the amplitude of the signal voltageapplied to the signal grid. The variable virtual cathode, in effect,acts like a source of electrons whose density varies at the signalfrequency. The positive output electrode draws these electrons, anddevelops a signal output voltage in its output impedance.

Now I have discovered. and made use of, an induction phenomenon whichoccurs within the tube, and specifically at the suppressor grid. If animpedance is inserted in the negative suppressor grid lead, it is foundthat signal voltage is developed across the impedance. This voltage isproduced by virtue of currents induced in the suppressor grid by thevirtual cathode of varying intensity.

Accordingly, it can be stated to be one of the main objects of mypresent invention to provide a method of, and means for, deriving signalvoltage from an amplifier, the method including establishing a virtualcathode effect within the amplifier, and inducing signal currents in anelectrode located between the virtual cathode and output electrode, thedesired signal voltage being developed by a load impedance in the pathof the induced currents.

Another important object of this invention is to insert in the negativesuppressor grid lead of a pentode amplifier a resonant circuit tuned toa desired signal frequency, and signal voltage being derived from theresonant circuit by virtue of signal currents induced in the suppressorgrid by the electrons flowing to the amplifier output electrode.

Another object of this invention is to provide a signal transmissiontube having in the order named a cathode, a signal grid, a positivescreen grid, a negative suppressor grid and a positive plate, a signaloutput circuit being coupled to the plate, said suppressor grid beingfree of any external coupling to the input or output circuits, and asignal-tuned circuit being included in the suppressor grid circuit todevelop signal voltage from signal currents induced in the suppressorgrid by the fiow of electrons to the plate.

Still another object of the invention is to derive signal voltage from asignal amplifier tube having input and output electrodes by inducingsignal currents in an auxiliary electrode located in the electron streamto the output electrode, and the auxiliary electrode being free of anycoupling to the other electrodes externally of the tube.

Still other objects of my invention are to improve generally theefficiency of signal amplifier circuits, and more especially to providea signal amplifier capable of simultaneous use as a source of amplifiedautomatic gain control voltage.

The novel features which I believe to be characteristic of my inventionare set forth in particularity in the appended claims; the inventionitself, however, as to both its organization and method of operationwill best be understood by reference to the following description takenin connection with the drawing in which I have indicateddiagrammatically several circuit organizations whereby my invention maybe carried into effect.

In the drawing Fig. 1 shows an amplifier with means for obtaining acontrol voltage from the suppressor grid circuit; Fig. 2 shows switchingmeans in the suppressor grid circuit for varying the selectivity of theamplifier; Fig. 3 shows a modification of Fig. 2; and Fig. 4 shows howautomatic selectivity control may be obtained with the presentinvention.

Referring now to the accompanying drawing, the invention is illustratedby embodiment in the intermediate frequency, (I. F.) amplifier oi. asuperheterodyne receiver. The amplifier may consist of a tube I, as forexample a pentode tube of the 6K? type, having a cathode 2, signal grid3, screen grid 4, suppressor grid 5 and output plate, or anode,electrode 6. The cathode is connected to ground by the usual self-biasresistor l, the latter being bypassed for signal currents.

The signal grid 3 is connected to the high potential side of theintermediate frequency input circuit 8, while the low potential side ofthe input circuit is returned to ground through the condenser 9'. Thenumeral l0 denotes a resonant network, also tuned to the intermediateIrequency, which is to be understood as being coupled to any source ofintermediate frequency voltage. Those skilled in the art know thatsuperheterodyne receivers usually embody a signal collector which feedssignals to one or more tunable radio frequency amplifiers. then appliesthe amplified signals to a first detector tube,-and upon the detectortube are also impressed locally reproduced oscillations derived from atunable local oscillator. The circuit It! may be located between outputelectrodes of the first detector, or it may be the output circuit of aprior intermediate frequency amplifier. Assuming that the receiver is ofthe broadcast type it can be varied over a range of 550 to 1500kilocycles; the intermediate frequency may be chosen from a range of 75to 450 kilocycles, and specifically it is preferred to use the 450kilocycle value.

The numeral H designates the resonant output circuit of the amplifier,and this circuit is similarly tuned to the operating intermediatefrequency. The plate 6 is connected through the coil of the outputcircuit II to a source of positive potential. The intermediatefrequencytuned circuit I2 derives the amplified intermediate frequencyvoltage from circuit II, and impresses the amplified voltage upon asecond detector. The demodulated voltage is then applied to one or moreaudio amplifiers, and finally the amplified voltage is reproduced in anydesired type of loudspeaker. There is applied to the screen grid 4 apositive potential of the order of 100 volts, the screen grid lead beingbypassed to ground for signal currents by the condenser 13. Thesuppressor grid 5 is connected to ground through a path which includesthe lead l4 and a resonant circuit, the latter circuit comprising coill5 shunted by condenser IS. The circuit l5--l6 is tuned to the operatingintermediate frequency of the amplifier network.

As explained previously, the effect of the negative suppressor grid 5 isto create a virtual cathode effect between the screen grid 4 and thegrid 5. Since the grid 5 is returned to ground it is at a negativepotential with respect to screen 4. The intensity of the virtual cathodeis varied because of the variable signal voltage applied to the signalgrid 3. In other words, the density of the virtual cathode betweenscreen grid 4 and suppressorgrid 5 will vary at the frequency of thesignal voltage frequency. This means that electrons will fiow to theplate 6 through suppressor 5, and from the variable density virtualcathode. The passage of these electrons induces in the suppressor gridcircuit currents of signal frequency, and since the signal frequency isthe intermediate frequency there is caused to flow through thesuppressor grid circuit intermediate frequency currents. These currentsproduce across the resonant circuit l5--l6 an intermediate frequencyvoltage which can be utilized in any desired manner. This voltage is ofsubstantial amplitude, and is amplified with respect to the amplitude ofthe intermediate frequency voltage applied to signal grid 3. For a giveninput'signal voltage, say 1 volt, at grid 3, and with coils of like "Q"in the plate and suppressor circuits, the voltage developed across thesuppressor coil will be about 80% that developed across the plate coil.If a 1 volt signal were applied to grid 3 and volts were developedacross the plate coil, then about8 volts would be developed across thesuppressor coil l5. The Q of eachcoil; the Rp of the tube; bias onsignal grid, each affect the relative magnitude of voltages across thecoils.

The latter The electrical circuit and functions disclosed above may beput to many uses. One of the uses to which the signal voltage acrosscircuit l5--I6 can be put to is for automatically controlling the volumeof the receiver. Thus, the rectifier H, which may be of the diode type,has its anode l8 coupled to the high potential side of circuit l5-l6 bycondenser l9, while cathode 20 thereof is grounded. The diode loadresistor 2| is connected between anode l8 and ground. There is developedacross resistor 2| a uni-directional voltage which varies directly inamplitude with the intermediate carrier amplitude. The voltage is usedfor controlling the bias of the signal grid 3 of amplifier tube I. Forthis purpose, the direct current voltage connection is provided betweenthe low potential side of circuit 8 and the anode end of resistor 2|.

The lead 30 is designated by the letters "AVC to show that this is theautomatic volume control lead. The filter netw0rk'3l prevents thetransmission to the controlled tubes of pulsating voltage components.The action of the AVG connection is well known; and it is believedsufficient to point out that as the intermediate carrier intensityincreases above the desired level, the voltage across l5l6 increases dueto a greater flow of induced current in the suppressor grid circuit. Therectifier output voltage across resistor 2| then increases, and the gainof tube l is reduced to an extent sufficient to bring down the receivervolume to normal. The lead 30 can be connected to the signal grids ofprior signal transmission tubes, if desired.

It will be observed that the grid 5 is free of reactive coupling to anyof the electrodes exterthe rectified audio output of diode I'I could beamplified and then utilized.

Further uses of the circuit will be discussed with reference to Fig. 2.The circuit shown is that associated with tube I, except that the inputcircuit l0, output circuit l2 and diode load circuit have been omitted.It is to be noted that the signal voltage applied to grid 3 causes theplate current to vary. Current is high in the suppressor grid circuitwhen plate voltage is low, and vice versa. If l8--l5 and l' are tuned tothe frequency of input circuit 8 oscillation takes place between thesuppressor and plate provided the bias on grid 8 is of .the order ofzero to 4 or 5 volts. No external coupling is provided between circuit land circuit |5-l6. Oscillation voltage could then be taken oil fromeither, or both, of circuits l8-I5 and I.

At grid biases greater than that causing oscillations, say 5 to -8volts, the oscillations cease and regeneration takes place. Hence, inthat negative bias range the circuit acts as a regenerative amplifier.At biases above -8 volts regenerarion ceases, a d the circuit behaves asa normal amplifier. The arrow through resistor 1 denotes an adjustablebias source.

In the regenerative, non-oscillation state brought about by tuning thesuppressor load |5-l5 just above resonance it is found the selectivityand gain of the plate circuit I is considerably enhanced. Theselectivity is several 'times sharper, and the gain about doubled.Circuits I6-I5 and I' may have identical Q5. The resulting circuit isvery stable, and may be employed to advantage in amateur and communication type receivers as an intermediate frequency amplifier.

If the grid bias is such that the circuit is not regenerating it isfound that I6-I5 puts a dip in the resonance curve of circuit I, Inother words, with circuit I6-I5 omitted in Fig. 2, and izcuit I tuned tothe desired frequency, the circuit I will have a. single-peaked curve.The resonance curve will broaden out to a doublepeaked curve whencircuit I6--I5, tuned to the frequency of /circuits 8 and I, is insertedin the suppressor lead. Hence, assuming the circuit in Fig. 2 to be anI. F. amplifier, a simple band width control can be provided byconnecting a switch element 40 in shunt across the circuit Iii-l5.circuit'is sharp; the amplifier is broad when the switch is opened.

Fig. 3 shows a modified form of band width control mechanism wherein thesecondary circuit I" of output transformer 4| maybe used as thesuppressor tuned circuit. In this case when the transformer 4| isproperly phased, and with switch contact element 42 on contact tap 43,the amplifier resonance curve is broad since circuit I is thenfunctioning as the suppressor resonant circuit. Conversely, when element42 contacts tap M the amplifier is sharp.

Automatic selectivity control is readily provided by the presentinvention. Fig. 4 shows such a selectivity control pressor circuit 50 istuned to a frequency just above resonance. say 470 kilocycles (kc) whenthe I. F. circuits 8. I, I" are each tuned to the operating I. F. of 460kc. The AVC rectifier is the usual signal rectifier, and delivers adirect -1 current voltage over AVC lead 6| to the signal grid 3 ofamplifier tube I. The grid bias applied over lead 6| becomesincreasingly more negative as the carrier amplitude at the signalcollector (not shown) increases. The bias over lead 6| can be applied tothe signal grids of earlier tubes concurrently. Those skilled in theradio art are fully aware of the manner of constructing such AVCcircuits. The suppressor circuit 50 acts to aid the AVG action by virtueof the following functions. When the bias applied over lead 6| to grid 3is small (weak signal reception) the gain of tube I is high, and thecircuit of tube I is regenerative thereby enhancing the selectivity andgain of circuit I.

As the signal carrier amplitude increases the AVG bias increases andcauses regeneration to cease. This reduces the gain and selectivity atcircuit I. Finally, for very high carrier amplitudes, such as localreception, very high negative biases are applied to the signal grids.Not only would the gain of tube I be greatly reduced, but circuit 50would absorb energy and would pro duce a dip in the widened amplifierresonance curve. This is the desired condition of sensitivity andselectivity for strong signal reception. The dip in the resonance curvecould be smoothed out by a resistor across the coil of circuit 50, or byusing a coil of low Q in circuit 50. This provides-a simple and mosteconomical automatic selectivity circuit.

The suppressor grid current may be increased by the use of a slightpositive potential on the suppressor. For example, a voltage of theorder of zero to volts may be applied to the sup- When switch is closedthe amplifier circuit. The 'suppressor grid. Further, in the circuit ofFig. the plate circuit II may be safeguarded from loading by the diodeacross the suppressor tuned circuit I6-I5. This is readily accomplishedby biasing grid 3 so that tube I is regenerative,- when the diode IIloads the tuned circuit it may bring the regeneration tonon-regeneration.

While I have indicated and described several systems for carrying myinvention into effect, it will be apparent to one skilled in the artthat my invention is by no means limited to the part cular organizationsshown and described, but that many modifications may be made withoutdeparting from the scope of my invention, as set forth in the appendedclaims.

What I claim is:

1. In a signal amplifier circuit of the type including a tube providedwith at least a cathode, an output electrode, and a signal grid andpositive screen grid arranged in succession in the electron stream fromcathode to output electrode, a signal input circuit connected to thesignal grid, a signal output circuit connected to said output electrode,means for applying a positive potential to the output electrode, anauxiilary electrode in said tube adjacent said screen grid and outputelectrode; means establishing the auxiliary electrode at a negativepotential relative to cathode thereby to produce a virtual cathode at'apoint between the screen grid and output electrode, an impedance incircuit with the auxiliary electrode for developing voltage of signalfrequency from signal currents induced in said auxiliary electrode byelectrons flowing from the virtual cathode to the output electrode andgain control rectifier means, responsive to said induced signal voltage,coupled solely to said impedance.

2. In a signal amplifier circuit of the typecincluding a tube providedwith at least a cathode, an output electrode, and a signal grid andpositive screen grid arranged in succession in the electron stream fromcathode to output electrode, a signal input circuit connected to thesignal grid, means for deriving amplified signal voltage from saidoutput electrode, means for applying a positive potential to the outputelectrode, an auxiliary electrode to said tube intermediate said screengrid and output electrode, means establishing the auxiliary electrode ata negative potential relative to cathode thereby to produce a virtualcathode at a point between the screen grid and output electrode, animpedance in circuit with the auxiliary electrode for developing voltageof signal frequency from signal currents induced in said auxiliaryelectrode by electrons flowing from the virtual cathode to the outputelectrode means responsive solely to signal voltage across aid impedancefor regulating the amplifier gain.

3. In a signal amplifier circuit of the type including a tube providedwith at least a cathode, an output electrode, and a signal grid andpositive screen grid arranged in succession in the electron stream fromcathode to output electrode, a signal input circuit connected to thesignal grid, means for applying a positive potential to the outputelectrode, an auxiliary electrode in said tube adjacent said screen gridand output electrode, means establishing the auxiliary electrode at anegative potential relative to cathode thereby to produce a virtualcathode at a point between the screen grid and output electrode, animpedance in circuit with the auxiliary electrode for developing voltageof signal frequency from signal currents induced in said auxiliaryelectrode by electrons flowing from the virtual cathode to the outputelectrode, means connected solely across said impedance for rectifyingthe developed signal voltage and controlling the amplifier gain.

4. In a signal amplifier circuit of the type including a tube providedwith at least a cathode, an output electrode, and a signal grid andpositive screen grid arranged in succession in the electron stream fromcathode to output electrode, a signal input circuit connected to thesignal grid, means for applying a positive potential to the outputelectrode, an auxiliary electrode in said tube adjacent said screen gridand output electrode, means establishing the auxiliary electrode at anegative potential relative to cathode thereby to produce a virtualcathode at a point between the screen grid and output electrode, an"

impedance in circuit with the auxiliary electrode for developing voltageof signal frequency from signal currents ,induced in said auxiliaryelectrode by electrons flowing from the virtual cathode to the outputelectrode, said impedance consisting of a signal-tuned resonant circuitand means connected across solely said last resonant circuit forcontrolling the amplifier gain.

5. In a signal amplifier circuit of the type including a tube providedwith at least a cathode, an output electrode, and a signal grid andpositive screen grid arranged in succession in the electron stream fromcathode to output electrode, a signal input circuit connected to thesignal grid, means for applying a positive potential to the outputelectrode, an auxiliary electrode in said tube adjacent said screen gridand output electrode, an impedance in circuit with the auxiliaryelectrode for developing voltage of signal frequency from signalcurrents induced in said auxiliary electrode by electrons flowing from,the cathode to the output electrode, said impedance being a resonantcircuit tuned to the frequency of said input circuit, and rectifiermeans coupled solely to the latter to derive a control voltage forvarying the amplifier gain.

6. In a high frequency circuit, an electron discharge tube provided witha cathode, a positive anode and at least two control gridsarrangedsuccessively in the electron stream to the anode, means for establishingeach of said grids at negative direct current voltages relative to thecathode, a high frequency input circuit connected between the cathodeand the grid adjacent thereto, an output circuit, tuned to the inputcircuit frequency, connected to the anode, an auxiliary resonantcircuit, tuned to the input circuit frequency, connected to the secondgrid adjacent the anode, and each of said output and auxiliary circuitshaving currents of said input frequency flowing therein, a rectifiercoupled across said auxiliary resonant circuit to rectify high frequencycurrents flowing therethrough, and means responsive to the rectifiedcurrents for regulating the gain of said tube.

7. In a high frequency circuit, an electron discharge tube provided witha cathode, a positive anode and at least two control grids arrangedsuccessively in the electron stream to the anode, means for establishingeach of said grids at negative direct current voltages relative to theoathode, a high frequency input circuit connected between the cathodeand the grid adjacent thereto, an output circuit, tuned to the inputcircuit frequency, connected to the anode, an auxiliary resonantcircuit, tuned to the input circuit frequency, connected to the secondgrid adjacent the anode, and each of said output and auxiliary circuitshaving currents of said input frequency flowing therein, and gaincontrol rectifier means, responsive to high frequency currents flowingthrough solely said auxiliary circuit.

SEYMOUR HUNT.

